This invention relates to a detector, and in particular to a fire detector.
A fire detector typically includes a sensing element such as an optical smoke sensing element, a CO sensor for detecting carbon monoxide, or a pyroelectric sensor for detecting the presence of a flame. Fire detectors based on the sensing of a single fire product (smoke, CO or flame) have, in recent years, been superseded by detectors based on the sensing of two or more combustion products. One of the combinations that is gaining popularity is the use of a CO sensor to support the customary optical detection of visible smoke, giving potential advantages in increasing discrimination between fire and false alarm sources.
Unfortunately, a fire detector having two or more sensing elements has inherent problems, as the different sensing elements tend to decay with time differently, and age differently. Moreover, in order to incorporate two or more sensing elements, existing detector designs have to increase in complexity considerably, thereby leading to a reduction in reliability.
Another problem that exists is that each sensing element has to be calibrated independently during production, with the calibration results being stored by means such as a trim pot, a select on test (SOT) resistor, an EEPROM or a mechanical link. This becomes more awkward and complex when a detector includes two or more sensing elements.
The main problem with such a detector is, however, the cost of replacement, as the sensing elements are provided on a single detector base. Hence, if the detector includes an optical smoke sensor and a CO sensor, the entire detector has to be replaced when one of the sensing elements (usually the CO sensor which tends to decay much quicker than the optical smoke sensor) decays to such an extent that it can no longer reliably sense a fire.